This invention relates to steel strips or sheets having improved corrosion resistance with or without a paint coating and press workability, and more particularly, to such surface-treated steel strips for use in automobiles.
Among surface treated steel strips, zinc coated steel has found the widest variety of applications, for example, in automobiles, electric appliances, building material and the like because of its improved sacrificial corrosion prevention effect. Recently, the need for rust prevention has been increased in some applications and it has been desired to enhance the rust prevention of zinc coated steel. There has been the need for imparting heavy duty rust prevention to zinc coated steel because the rust prevention that current zinc-coated steel possesses is still insufficient in certain applications. More illustratively, zinc coated steel strips have poor phosphatability, paintability, and wet adhesion of paint coating, and deteriorate in corrosion resistance during service at joints such as hemmed joints as often formed in automobile doors whether or not they are coated with paint. A closer attention has been paid to these drawbacks and there is the strong desire to overcome them. Particularly, surface treated steel strips for use in automobiles are required to have improved corrosion resistance with or without paint coating, particularly improved perforation corrosion resistance at joints as well as good weldability, workability, phosphatability and paintability.
Among prior art conventional surface treated steel strips, there are known galvannealed steel strips which satisfy the above requirements to some extent as they possess exceptionally high corrosion resistance after paint coating. The galvannealed steel is prepared by subjecting steel to zinc hot-dipping or zinc electroplating followed by a heat treatment to form a Zn-Fe alloy coating having a major proportion of .delta..sub.1 phase. This process is well known for decades in the art and galvannealed steel often exhibits good corrosion resistance after paint coating. The need for heat treatment, however, detracts from the mechanical properties of steel strips and is unsuccessful in providing steel strips with such a high degree of strength and workability as is currently required for automobile use. Further, when thinly coated, galvannealed steel strips do not possess satisfactory local corrosion resistance or perforation corrosion resistance during service at joints like hemmed joints.
In order to eliminate the above-mentioned shortcomings of galvannealed steel while taking advantage of its excellent corrosion resistance with or without paint coating, Zn-Fe alloy electroplating has recently been used as an improvement over the galvannealing as disclosed in Japanese Patent Application Kokai Nos. SHO 54-107838, 57-60087 and 57-200589, and Japanese Patent Publication No. SHO 57-61831, for example. The Zn-Fe alloy electroplating is substantially equivalent to galvannealing in as to corrosion resistance with or without paint coating, paint adhesion, phosphatability and weldability where the content of iron is in the range of 5% to 30% by weight. Since these results are obtained by carrying out electroplating so as to achieve the properties of galvannealed coatings, it is not expectable that properties other than workability might exceed those of galvannealed coatings. It is believed that corrosion resistance which is most important among others is improved because the alloy is more noble in corrosion potential than pure zinc and thus exhibits a smaller potential difference with respect to the underlying steel. In addition, microcells form in the deposit as well as corrosion products contaminated with iron. These factors together result in a reduced rate of corrosion.